1. Technical Field
The present invention generally relates to a protection circuit for preventing voltage surges from being applied to a circuit and, more particularly, to such a protection circuit incorporated in a node (such as a line extender or a distribution amplifier) of a broadcast distribution system.
2. Description of Related Art
Broadband community area or cable television (CATV) systems have been designed with a system architecture known as "mink and feeder." The function of a trunk coaxial cable is to deliver broadband television signals from a reception center, or headend, over the shortest distance with the least amount of amplification to a plurality of distribution points. The distribution points are connected to feeder coaxial cables which emanate from the trunk coaxial cable and contain subscriber tap off devices. The broadband television signals are transmitted over the distribution network from the headend in a forward direction to a plurality of subscribers, which is generally termed a one-way system. However, more complex systems include a reverse signal path. The reverse channel is used for system control, messaging, pay-per-view events, monitoring, and the like.
As the signal travels down the feeder cable and through successive tap off devices, loss is incurred and the power level becomes progressively lower. If the last subscriber is reached before the signal level drops below the minimum tap port level set for the system, the feeder can be terminated and no additional devices are required. If, however, the signal level drops below the minimum required tap port level before the last subscriber (or subscribers) is reached, it is necessary to amplify the signal with a line extender amplifier before the feeder path can be continued. After amplification and equalization, the feeder is continued.
FIG. 1 is a general block diagram of a line extender which is usable in a television distribution system. Forward path communication signals (e.g., in a frequency range of 50-860 Megahertz) from a coaxial cable are supplied via a capacitor 101 to a diplex filter 103. Diplex filter 103 provides these forward path communication signals to a series of amplifiers 105, 107, 109, and 111. Amplifiers 105, 107, 109, and 111 may, for example, be Gallium Arsenide (GaAs) Field Effect Transistor (FET) amplifier circuits. Equalization circuits 113 and 115 are provided to equalize the forward path communication signals. In one implementation, equalization circuit 113 may be a plug-in circuit so that different circuits may be easily incorporated into the line extender depending on the signal characteristics at the particular location of the line extender. The output of amplifier 111 is supplied to a diplex filter 117 and output to the distribution network via a capacitor 118. Reverse path communication signals (e.g., in a frequency range of 4-160 Megahertz) are supplied to diplex filter 117 via capacitor 118. These signals are then provided to reverse path amplifier 119. The output of reverse path amplifier 119 is supplied to the distribution system via diplex filter 103 and capacitor 101. AC power (60 Volts, 60 Hertz) or higher, passes through a path including inductors 121 and 123. Jumper Connectors 125 and 127 are provided to direct power flow through the amplifier housing as required by the system's architecture.
Three types of signals (i.e., forward path communications signals, reverse path communication signals, and AC power) are present in line extenders such as the line extender of FIG. 1, as well as in other amplifiers in the television distribution network. Voltage surges associated with any of these signals can cause damage to the active components of the line extenders and other amplifiers, especially during installation procedures or during "hot coring." (Hot coring is the procedure of striping out the outer conductor of a coaxial line using a conductive blade, while AC power is present.) In particular, damage of amplifiers 109 and 111 shown in FIG. 1 has been observed when a coaxial cable of the distribution system is connected to the output terminal for the forward path amplifiers of line extenders. This damage is caused by high frequency surges generated by the arcing which occurs as the connection or disconnection is made. While various surge protection circuits are known, these circuits generally utilize one or more capacitors in the signal path, thereby resulting in a DC disconnection in the signal path. Such a DC disconnection can in some cases disrupt the biasing of the amplifiers. In order to provide protection against surges of both positive and negative polarities, surge protection circuits generally require the use of two power supplies (e.g., a positive and a negative power supply). The need for two power supplies increases the costs associated with the line extenders. Protection circuits which do not require biasing operate either at low level (system inputs) or at considerably high levels which are beyond the damage threshold of the GaAs FETs. Such circuits are also generally slow in responding to such surges.
Accordingly, it would be desirable to provide a protection circuit which overcomes these and other disadvantages of prior systems.